Growth of plutons by floor subsidence: implications for rates of emplacement, intrusion spacing and melt-extraction mechanisms

Geophysical and field-based studies indicate that granitic phtons occur as either tabular (disk) or wedge (funnel) shapes whose length (L) to thickness (7) ratio is controlled by the empirical power law, T = 0.6(+0.15)L0.6(~.l). The dimensions of phtons are selfsimilar to other natural subsidence phenomena (calderas, ice cauldrons, sinkholes, ice pits) and it is proposed that they grow in a similar fashion by withdrawal of material (melt) from an underlying source, which is then transferred to the growing pluton within the crust. Experimental studies show that growth of subsidence structures occurs by vertical inflation >> horizontal elongation of an initial depression with L = width of the source region. If pluton growth is modelled in the same way, the empirical power law relating T and L defines limits for phton growth that are imposed by the width, thickness and degree of partial melting from a lower crustal source. Several growth modes that predict testable internal structural patterns are identified for plutons, depending on whether they are tabular or wedge-shaped, grow by continuous or pulsed magma delivery and whether magma is accreted from bottom to top, or vice versa. Rates of pluton growth are geologically fast (hundreds to hundreds of thousands of years) if magma supply is effectively continuous, but can also take millions of years if the time between magma delivery events is much longer than magma injection events. Plutons formed by melt extraction from an area directly beneath require large degrees of partial melting and or very thick sources. Lower degrees of partial melting and thinner sources are permitted when melt extraction occurs over a larger region, which can lead to the formation of spaced plutons. Tabular pluton growth will tend to favour widely spaced plutons, unless degrees of partial melting in the source are high. Wedge-shaped plutons can form much closer together and require lower degrees of partial melting. These results are in general agreement with current geophysical, petrological Correspondence to: A. R. Cruden and experimental estimates of partial melting in the lower continental crust

Vein development during folding in the upper brittle crust: The case of tourmaline-rich veins of eastern Elba Island, northern Tyrrhenian Sea, Italy.

Detailed structural analysis of tourmaline-rich veins hosted in the contact aureole of the 6 Ma Porto Azzurro granite in southeastern Elba Island, northern Tyrrhenian Sea is presented. Using geometric features of the veins, the physical conditions at the time of vein formation are estimated, namely the stress ratio, driving stress ratio and fluid overpressure. Two vein sets (A veins and B veins) have been recognized based on orientation and thickness distributions and infilling material. Analysis of vein pole distributions indicates stress ratio = 0.57 and driving stress ratio = 0.24 for the A veins andstress ratio = 0.58 and driving stress ratio = 0.47 for the B veins, and fluid pressures less than the intermediate stress magnitude. Analysis of geometric features of the veins gives estimated fluid overpressuresof between 16 MPa (A veins) and 32 MPa (B veins). We propose a model for the tectonic environment of vein development, in which formation of secondary permeability in the deforming thermal aureole of the Porto Azzurro pluton was controlled by ongoing development of fracture systems in the hinge zone of a regional NNW-SSE trending fold that favored transport and localization of hydrothermal fluids

Interactions between low-angle normal faults and plutonism in the upper crust: Insights from the island of Elba, Italy: Comment

The interplay between magmatism and tectonics is pivotal for deciphering the tectonic and geodynamic evolution of mountain belts. In the northern Apennines - northern Tyrrhenian Sea orogenic system large amounts of magma were emplaced at shallow crustal levels since the middle-late Miocene to recent (e.g. Serri et al., 1993; Rosenbaum et al., 2008). In this sector of the Apennine chain the mutual relationships between magmatism and tectonics are still a matter of debate (e.g., Rossetti et al., 1999; Acocella and Rossetti, 2002; Musumeci et al., 2005; 2008; Rosenbaum et al., 2008). Elba Island, located in the Tyrrhenian Sea about 30 km west of the inner portion of the chain, is a key area where tectonics - magmatism relationships can be studied

Geometric scaling of tabular igneous intrusions: implications for emplacement and growth

The horizontal (L) and vertical (T) dimensions of broadly tabular, sub-horizontal intrusions of mafic to felsic composition emplaced into shallow to mid-crustal levels of continental crust reveal two well-defined and continuous curves in log L vs. log T space. The data set spans six and five orders of magnitude in L (1 m to 1000 km) and T (10 cm to 10 km), respectively. Small tabular sheets and sills (mafic and felsic) define a straight line with a slope ~ 0.5 at all horizontal length scales, similar to the known geometric scaling of mafic dikes, indicating that the L/T ratio of these intrusions to increases with increasing L (horizontal lengthening dominates over vertical thickening). Laccoliths, plutons, layered mafic intrusions and batholiths define an open, continuous S-shaped curve that bifurcates from the tabular sheets and sills curve at L ~ 500 m towards higher T values. For L ~ 0.5 to 10 km the slope of this curve is ~ 1.5, corresponding to laccoliths that are characterized by a decrease in L/T ratio with increasing L (vertical thickening dominates over horizontal lengthening). Between L ~ 10 and 100 km the slope has a mean value ~ 0.8, indicating that plutons and layered mafic intrusions have a tendency for horizontal lengthening over vertical thickening as L increases. Batholiths and very large layered mafic intrusions with L > 100 km lie on a slope ~ 0 with a threshold thickness ~ 10 km. The continuous nature of the dimensional data over such a wide range of length scales reflects a spectrum of igneous emplacement processes repeated in space and time. We discuss how thresholds and transitions in this spectrum, defined by bifurcations between the curves (e.g., between sill and laccolith emplacement) and changes in slope, largely reflect depth- and time-dependent changes in emplacement mechanisms rather than factors such as magma viscosity, composition and temperature

Isotope evidence for Archean accordion-tectonics in the Superior Province

Archean cratons preserve the oldest continental crust, but their tectonic mode of formation and assembly remain key unknowns. The Superior Province is Earth’s largest Archean craton and comprises Eo-Neoarchean plutonic-gneiss terranes separated by Meso-Neoarchean granite-greenstone terranes and metasedimentary belts. Resolving whether the plutonic-gneiss terranes represent dismembered fragments of a once contiguous proto-craton or a series of unrelated accreted crustal fragments is critical to understanding the early evolution of the Superior Province and Archean tectonics. We present zircon U-Pb-Hf isotopic data from the Tannis and Cedar Lake TTG gneisses, which record the early crustal evolution of the Winnipeg River plutonic-gneiss terrane and are some of the oldest rocks in the western Superior Province. The gneisses yield a large range of zircon εHf(t) signatures (−6 to +4) at igneous formation (ca. 3.3–3.25 Ga) indicating coeval crustal growth and reworking of isotopically depleted and evolved sources. Crustal reworking of Eo-Paleoarchean sources is supported by sub-chondritic ca. 3.5–3.4 Ga zircon xenocrysts in the Cedar Lake gneiss. The early crustal evolution of the central Winnipeg River terrane is similar to the Hudson Bay and Minnesota River Valley terranes, on the northern and southern margins of the Superior Province, respectively. Correlation of the early history amongst the three plutonic-gneiss terranes supports a tectonic model in which a once coherent Eo-Paleoarchean proto-craton disaggregated into three fragments during formation of intervening granite-greenstone terranes in the Mesoarchean. These fragments reaggregated in the Neoarchean. The complex history of the Superior Province highlights that Archean cratons are not simple entities that formed in a single event, but may have experienced times of cratonic breakup and reassembly.J.W.D. Strong, J.A. Mulder, P.A. Cawood, A.R. Cruden, O. Nebe